In the production of various components such as those used in automobiles, home appliances, power tools, hardware and other high volume mechanical, electro-mechanical and electrical products, it is often desirable to create high precision smooth shaft surfaces. Such surfaces are typically utilized in conjunction with press fit bearings, sleeve bearing journals, controlled slide fitments, rotating element mountings, structural press fitments and other precise connections of mechanical elements to shaft-like parts.
Where such press fits and other precise connections are currently made into the interfaces of ball bearings, or other hardened elements, the surface of the shaft to be fitted is generally turned or drawn to a pregrind diameter, and then final sized by grinding the shaft on center or, alternatively, by centerless grinding. Typical grinding machinery is large and expensive for either process. Additionally, this grinding process is relatively slow and the grinding wheels require regular dressing to maintain their size and surface shape. The grinding process also creates chips and swarf that must be disposed of under controlled conditions, thus adding to shaft production expense and slowing production time. When a highly polished finish for the shaft surface is also required, such as in a high speed journal bearing application, subsequent polishing or other finish grinding is also necessary which also adds to process costs and time.
Less precise press fits of shafts into gears, laminations, commutators and similar rotating elements usually entail the use of straight or diamond pattern knurls. Knurled surfaces are generally created by cylindrical die rolling utilizing a rolling attachment in a turning machine or in a cylindrical die rolling machine that is employed subsequent to a lathe turning operation. Typically, the tolerance of the outside diameter of the knurl is less precise than the previously turned surface. However, since the surface is deformable and is generally concentric, it is acceptable for lower precision applications. Some more precise applications utilize a special straight knurl which, when rolled on the outside diameter of the shaft, more precisely follows the initial diameter of the shaft and when rolled on high precision surfaces can produce tolerances and concentricities which more closely approximate the original shaft surface dimension. In these high precision knurling operations, the rolling is frequently performed on a surface that has been previously ground. For additional precision, some knurled surfaces are actually ground subsequent to rolling. These surfaces are suitable for press fitting into hardened bores or other similar not easily-deformed elements.
To reduce shaft processing costs, and to eliminate the generation of chips and swarf, various cold work processes have been employed involving the swaging of the journal and press fit areas on shafts to the required precision. However, precision and size repeatability have not proven accurately controllable to desired high tolerances with such swaging processes.
Similarly, direct roll sizing of the shaft using a cylindrical die rolling machine has been attempted for shafts of cold finished steel having an original diameter tolerance range of 0.002 inch (50 microns). However, even the stiffest rolling machines and the highest grade dies have exhibited spring-back characteristics and run out, respectively, that limit diametral repeatability and adjustability of the rolled surfaces. Hence, the tolerance obtained from a conventional cylindrical die rolling process is limited.
It is therefore an object of this invention to provide an improved method and apparatus for forming high production shaft-like parts with precision surfaces. The method and apparatus should entail little or no generation of chips or swarf and should enable formation of surfaces having increased tolerances for high precision applications. Such tolerances should preferably be within the range of approximately 0.0003 inch (8 microns). This tolerance range should be highly repeatable and size adjustability should be contemplated according to this method and apparatus.